Subband-based demodulation for an OFDM-based communication system

ABSTRACT

For subband-based OFDM demodulation, a “partial” Fourier transform is performed on a sequence of N input samples for an OFDM symbol to obtain N c  received symbols for a group of N c  data subbands, where N c ·L=N and L&gt;1. For the partial Fourier transform, the N input samples are rotated with a phasor to obtain N rotated input samples, which are accumulated (for each set of L samples) to obtain N c  time-domain values. An N c -point FFT is performed on the NC time-domain values to obtain the N c  received symbols. Channel gain estimates for the data subbands are also obtained, for example, by performing a partial Fourier transform to obtain received pilot symbols, an inverse FFT to obtain time-domain channel gain values, and an FFT to obtain channel gain estimates for the data subbands. The received symbols are processed with (e.g., equalized by) the channel gain estimates to obtain recovered data symbols.

I. FIELD

The present invention relates generally to communication, and morespecifically to techniques for performing demodulation in an orthogonalfrequency division multiplexing (OFDM) based communication system.

II. BACKGROUND

OFDM is a modulation technique that effectively partitions the overallsystem bandwidth into a number of (N) orthogonal subbands. Each subbandis associated with a respective subcarrier that may be modulated withdata. The subbands are also commonly referred to as tones, subcarriers,bins, and frequency channels.

OFDM is widely used in various communication systems. For example, anorthogonal frequency division multiple access (OFDMA) system utilizesOFDM and can support multiple users. The N subbands may be used for dataand pilot transmission in various manners, depending on the systemdesign. For example, the OFDMA system may partition the N subbands intomultiple disjoint groups of subbands and allocate each subband group toa different user. Multiple users can then be supported simultaneouslyvia their assigned subband groups.

In many instances, it is only necessary to demodulate a subset of the Nsubbands in an OFDM-based system. A straightforward method to process asubset of the N subbands is to perform an N-point fast Fourier transform(FFT) on time-domain samples to obtain frequency-domain symbols for allN subbands. The symbols for the subbands of interest are then extractedand processed, and the symbols for all other subbands are discarded.This straightforward method requires memory storage proportional to theN subbands and further requires computation for all N subbands, even ifonly a small subset of the N subbands is used for data transmission.

There is therefore a need in the art for techniques to more efficientlyperform demodulation in an OFDM-based system when only a subset of the Nsubbands is used for data transmission.

SUMMARY

Techniques for performing subband-based OFDM demodulation are describedherein. These techniques allow a receiver to perform processing for onlythe subbands of interest instead of all N subbands.

In one aspect, techniques for performing “partial” Fourier transform forN_(c) subbands among N total subbands, where N>N_(c)>1, are described.The N_(c) subbands include every L-th subband among the N totalsubbands, where N_(c)·L=N. To compute the partial Fourier transform forthe N_(c) subbands consisting of subbands m, m+L, and so on, a sequenceof N input samples is rotated (by multiplying each input sample with aphasor$\left. {W_{N}^{mn} = {\mathbb{e}}^{{- j}\frac{2\pi\quad{mn}}{N}}} \right)$to obtain a sequence of N rotated input samples. The sequence of Nrotated input samples is then accumulated, for each set of L rotatedinput samples, which are spaced N_(c) samples apart, to obtain asequence of N_(c) time-domain values. An N_(c)-point fast Fouriertransform (FFT) is then performed on the sequence of N_(c) time-domainvalues to obtain N_(c) frequency-domain values for the N_(c) subbands.The partial Fourier transform provides the frequency-domain values forthe N_(c) subbands using an N_(c)-point FFT instead of an N-point FFT.

In another aspect, techniques for performing channel estimation in anOFDM-based system are described. For this system, pilot symbols aretransmitted on subbands in group p, and channel gain estimates forsubbands in group m are desired. For the channel estimation, a partialFourier transform is first performed on the sequence of input samples toobtain received pilot symbols for the subbands in group p. Channel gainestimates for the subbands in group p are then obtained by removing themodulation on the received pilot symbols. An IFFT is next performed onthe channel gain estimates for group p to obtain time-domain channelgain values, which may be derotated with a phasor W_(N) ^(−pn) to obtainderotated time-domain channel gain values. To obtain channel gainestimates for group m, the derotated time-domain channel gain values arerotated with a phasor W_(N) ^(mn) to obtain rotated channel gain valuesfor group m. The time-domain channel gain values may also be rotatedwith W_(N) ^((m−p)n) to directly obtain the rotated channel gain valuesfor group m. In any case, an FFT is performed on the rotated channelgain values to obtain channel gain estimates for the subbands in groupm. Channel gain estimates for other groups of subbands may be obtainedby processing the (derotated) time-domain channel gain values in similarmanner, albeit with different phasors.

In yet another aspect, techniques for performing subband-baseddemodulation in the OFDM-based system are described. A partial Fouriertransform is performed on the sequence of input samples to obtainreceived symbols for a group of subbands. Channel gain estimates for thegroup of subbands are also obtained. The received symbols are thenprocessed with (e.g., equalized by) the channel gain estimates to obtainrecovered data symbols for the group of subbands. Demodulation for othergroups of subbands may be performed in similar manner.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and nature of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify correspondingly throughout and wherein:

FIG. 1A shows an OFDM subband structure;

FIG. 1B shows a subband arrangement for an OFDM-based system;

FIG. 2 shows a process for performing subband-based OFDM demodulation;

FIG. 3 shows a process for performing partial Fourier transform;

FIG. 4 shows a process for performing channel estimation;

FIG. 5 shows a transmitter in the OFDM-based system;

FIG. 6 shows a receiver in the OFDM-based system;

FIG. 7 shows a partial Fourier transform unit for one group of subbands;

FIG. 8 shows a channel estimator; and

FIG. 9 shows a subband-based OFDM demodulator.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs. For clarity, in thefollowing description, “sequence” is used for time-domain samples andvalues.

FIG. 1A shows an OFDM subband structure. An OFDM-based system has anoverall system bandwidth of W/N MHz, which is partitioned into Northogonal subbands 110 using OFDM. Each subband has a bandwidth of WINMHz and is associated with a respective carrier 112 that may bemodulated with data. The OFDM-based system may use only the centersubbands for data and pilot transmission and reserve some subbands onthe two ends as guard subbands to allow the system to meet spectral maskrequirements. For simplicity, the following description assumes that allN subbands are used for transmission.

FIG. 1B shows an exemplary frequency division multiplex (FDM) scheme forpartitioning the N subbands in the OFDM-based system. For this FDMscheme, the N subbands are arranged into L disjoint groups, with eachgroup including N_(c) subbands, where N_(c)·L=N. For example, theOFDM-based system may have N=4096 total subbands and L=8 subband groups,with each group including N_(c)=512 subbands. The L groups are disjointin that each of the N subbands belongs to only one group. The N_(c)subbands in each group are uniformly distributed across the N subbandssuch that consecutive subbands in the group are spaced apart by Lsubbands. The subbands in each group are thus interlaced with thesubbands in the other L-1 groups. The N subbands may be partitioned inother manners. For simplicity, the following description assumes thesubband arrangement shown in FIG. 1B.

With OFDM, one modulation symbol for either data or pilot may betransmitted on each of the N subbands in each symbol period. A datasymbol is a modulation symbol for data, and a pilot symbol is amodulation symbol for pilot. If fewer than N subbands are used fortransmission, then a signal value of zero is provided for each unusedsubband. For each symbol period, N symbols (i.e., data symbols, pilotsymbols, and/or zeros) for the N subbands are transformed to the timedomain using an N-point inverse fast Fourier transform (IFFT) to obtaina transformed symbol that contains N time-domain chips. To combatinter-symbol interference (ISI), a portion of each transformed symbolmay be repeated to form a corresponding OFDM symbol that contains N+Cchips, where C is the number of chips being repeated. The repeatedportion is often referred to as a cyclic prefix. The OFDM symbol is thentransmitted over a communication link.

A receiver obtains N+C input samples for the OFDM symbol, where eachinput sample corresponds to a transmitted chip. The receiver removes Cinput samples for the cyclic prefix and obtains a sequence of N inputsamples for the transformed symbol. The receiver may then perform anN-point fast Fourier transform (FFT) on the N input samples to obtain Nfrequency-domain received symbols for the N subbands. The receivedsymbols may be expressed as:X(k)=H(k)S(k)+N(k), for k=0 . . . (N−1),  Eq (1)where S(k) is the symbol transmitted on subband k;

-   H(k) is the complex channel gain for subband k;-   X(k) is the symbol received on subband k; and-   N(k) is the noise at the receiver for subband k.

The receiver may recover the transmitted data symbols as follows:$\begin{matrix}{{{\hat{S}(k)} = {\frac{X(k)}{\hat{H}(k)} \approx {{S(k)} + {\overset{\sim}{N}(k)}}}},{{{for}\quad k} = {0\quad\ldots\quad\left( {N - 1} \right)}},} & {{Eq}\quad(2)}\end{matrix}$where Ĥ(k) is an estimate of the channel gain for subband k;

-   Ŝ(k) is an estimate of the symbol transmitted on subband k; and-   Ñ(k) is the post-processed noise.    Equation (2) indicates that the data symbol S(k) transmitted on    subband k may be recovered by dividing the received symbol X(k) for    subband k by the channel gain estimate Ĥ(k) for subband k. This    operation is commonly referred to as equalization. The receiver may    estimate the channel gains based on pilot symbols transmitted by the    transmitter.

If the receiver only needs to recover data on one or a few groups ofsubbands, then it is more efficient to perform processing for only thesubbands of interest instead of all N subbands. The gain in efficiencyis especially pronounced when N is large (e.g., N=4096). The transmittermay transmit pilot symbols on a group of subbands different from thegroups of subbands used for data transmission. In this case, thereceiver can estimate the channel gains for the data subbands (i.e.,subbands used for data transmission) based on pilot symbols received onthe pilot subbands (i.e., subbands used for pilot transmission).

FIG. 2 shows a flow diagram of a process 200 for performingsubband-based OFDM demodulation to recover data symbols transmitted onone group of subbands (group m) using pilot symbols transmitted onanother group of subbands (group p). For the FDM scheme shown in FIG.1B, group m includes subbands Lk+m, for k=0 . . . (N_(c)−1), and group pincludes subbands Lk+p, for k=0 . . . (N_(c)−1), where 0≦m≦(N_(c)−1) and0≦p≦(N_(c)−1).

Initially, a partial Fourier transform is performed on the sequence of Ninput samples to obtain a group of N_(c) received symbols for thesubbands in group m (step 210). The sequence of input samples is denotedas {x(n)}, which is x(n) for n=0 . . . (N−1). The group of receivedsymbols is denoted as {X_(m)(k)} or {X (Lk+m)}, which is X(Lk+m) for k=0. . . (N_(c)−1). The partial Fourier transform utilizes an N_(c)-pointFFT instead of an N-point FFT and may be performed as described below.

The sequence of N input samples is also processed to obtain channel gainestimates for the subbands in group m (step 220). The channel gainestimates for group m are denoted as {Ĥ_(m)(k)} or {Ĥ(Lk+m)}, which isĤ(Lk+m), for k=0 . . . (N_(c)−1). Step 220 may be performed with apartial Fourier transform and an inverse Fourier transform, as alsodescribed below. The received symbols for the subbands in group m arethen processed with the channel gain estimates for the subbands in groupm to obtain recovered data symbols for the subbands in group m, e.g., asshown in equation (2) (step 230). The recovered data symbols for group mare denoted as {Ŝ_(m)(k)} or {Ŝ(Lk+m)}, which is Ŝ(Lk+m), for k=0 . . .(N_(c)−1).

The Fourier transform for the N, subbands in group m may be expressedas: $\begin{matrix}{\begin{matrix}{{X\left( {{Lk} + m} \right)} = {\sum\limits_{n = 0}^{N - 1}{{x(n)} \cdot W_{N}^{{({{Lk} + m})}n}}}} \\{= {\sum\limits_{n = 0}^{N - 1}{{x(n)} \cdot W_{N}^{mn} \cdot W_{N_{c}}^{kn}}}}\end{matrix},{{{for}\quad k} = {0\quad\ldots\quad\left( {N_{c} - 1} \right)}},} & {{Eq}\quad(3)}\end{matrix}$where $W_{N}^{\alpha} = {\mathbb{e}}^{{- j}\frac{2{\pi\alpha}}{N}}$and x(n) is the input sample for sample period n. The following termsmay be defined:{tilde over (x)} _(m)(n)=x(n)·W _(N) ^(mn), for n=0 . . . (N−1), and  Eq(4) $\begin{matrix}{{{g_{m}(n)} = {\sum\limits_{i = 0}^{L - 1}{{\overset{\sim}{x}}_{m}\left( {n + {N_{c} \cdot i}} \right)}}},{{{for}\quad n} = {0\quad\ldots\quad\left( {N_{c} - 1} \right)}},} & {{Eq}\quad(5)}\end{matrix}$where {tilde over (x)}_(m)(n) is a rotated input sample obtained byrotating the input sample x(n) by${W_{N}^{mn} = {\mathbb{e}}^{{- j}\frac{2\pi\quad{mn}}{N}}},$which is a phasor that varies from sample to sample; and g_(m)(n) is atime-domain value obtained by accumulating L rotated input samples thatare spaced apart by N_(c) samples.

Equation (3) may then be expressed as: $\begin{matrix}{{{X_{m}(k)} = {{X\left( {{Lk} + m} \right)} = {\overset{N_{c} - 1}{\sum\limits_{n = 0}}{{g_{m}(n)} \cdot W_{N_{c}}^{kn}}}}},{{{for}\quad k} = {0\quad\ldots\quad{\left( {N_{c} - 1} \right).}}}} & {{Eq}\quad(6)}\end{matrix}$

FIG. 3 shows a flow diagram of a process 210 a for performing partialFourier transform to obtain received symbols for one group of subbands.Process 210 a may be used for step 210 in FIG. 2. Initially, thesequence of N input sample x(n) is rotated by multiplying each inputsample by W_(N) ^(mn) to obtain a sequence of N rotated input samplesfor group m, which is denoted as {{tilde over (x)}_(m)(n)}, as shown inequation (4) (step 312). The sequence of N rotated input samples is thenaccumulated, in sets of L rotated input samples, to obtain a sequence ofN_(c) time-domain values for group m, which is denoted as {g_(m)(n)}, asshown in equation (5) (step 314). Each set includes every N_(c)-thsamples in the sequence of rotated input samples, with the N_(c) setsbeing associated with different starting rotated input samples in thesequence. An N_(c)-point FFT is then performed on the sequence of N_(c)time-domain values to obtain N_(c) received symbols for group m, asshown in equation (6) (step 316).

FIG. 4 shows a process 220 a for obtaining channel gain estimates forthe subbands in group m based on pilot symbols received on the subbandsin group p, where p≠m. Process 220 a may be used for step 220 in FIG. 2.Initially, the received pilot symbols for the subbands in group p areobtained, for example, using process 210 a described above forrecovering the data symbols for the subbands in group m (step 412). Theoutput of step 412 is N_(c) received pilot symbols, which are denoted asX(Lk+p) or {circumflex over (P)}(Lk+p), for k=0 . . . (N_(c)−1).

The modulation on the received pilot symbols is then removed to obtainchannel gain estimates for the subbands in group p (step 414), asfollows:Ĥ _(p)(k)=Ĥ(Lk+p)={circumflex over (P)}(Lk+p)·P*(Lk+p), for k=0 . . . (N_(c)−1),  Eq (7)where P(Lk+p) is the known pilot symbol for subband k in group p. Thechannel gain estimates for group p are denoted as {Ĥ_(p)(k)} or{Ĥ(Lk+p)}, which is Ĥ(Lk+p) for k=0 . . . (N_(c)−1). An N_(c)-point IFFTis then performed on the channel gain estimates for group p to obtain asequence of N_(c) time-domain channel gain values, {h_(p)(n)}, whichconsist of modulated time-domain channel estimates, h_(p)(n)=h(n)·W_(N)^(pn), for n=0 . . . (N_(c)−1) (step 416). The sequence of N_(c)time-domain channel gain values is then derotated by multiplication withW_(N) ^(−pn) to obtain a sequence of N_(c) derotated time-domain channelgain values, as follows: h(n)=h_(p)(n)·W_(N) ^(−pn), for n=0 . . .(N_(c)−1) (step 418).

The channel gain estimates for the subbands in group m are then derivedfrom the sequence of derotated time-domain channel gain values. TheFourier transform for the derotated channel gain estimates for N_(c)subbands may be expressed as: $\begin{matrix}{{{\hat{H}(k)} = {\sum\limits_{n = 0}^{N_{c} - 1}{{h(n)} \cdot W_{N}^{kn}}}},{{{for}\quad k} = {0\quad\ldots\quad{\left( {N_{c} - 1} \right).}}}} & {{Eq}\quad(8)}\end{matrix}$The Fourier transform for the channel gain estimates for the subbands ingroup m may be expressed as: $\begin{matrix}\begin{matrix}{{{\hat{H}}_{m}(k)} = {\hat{H}\left( {{Lk} + m} \right)}} & \quad \\{= {\sum\limits_{n = 0}^{N_{c} - 1}{{h(n)} \cdot W_{N}^{{({{Lk} + m})}n}}}} & {,{{{for}\quad k} = {0\quad\ldots\quad\left( {N_{c} - 1} \right)}},} \\{= {\sum\limits_{n = 0}^{N_{c} - 1}{{h(n)} \cdot W_{N}^{mn} \cdot W_{N_{c}}^{kn}}}} & \quad\end{matrix} & {{Eq}\quad(9)}\end{matrix}$As indicated in equation (9), the channel gain estimates for thesubbands in group m can be obtained by first multiplying the sequence ofderotated time-domain channel gain values, {h(n)}, by W_(N) ^(mn) toobtain a sequence of rotated channel gain values (step 420). AnN_(c)-point FFT is then performed on the sequence of rotated channelgain values to obtain the channel gain estimates for the subbands ingroup m (step 422). The derotation in step 418 and the rotation in step420 may be combined, and the rotated channel gain values for group m maybe obtained as h_(m)(n)=h_(p)(n)·W_(N) ^((m−p)n).

FIG. 5 shows a block diagram of a transmitter 500 capable oftransmitting data on one or more groups of subbands. For clarity, thefollowing description is for data transmission on M groups of subbands(i.e., groups a through m) and pilot transmission on one group ofsubbands (i.e., group p).

At transmitter 500, an encoder/modulator 514 receives traffic data froma data source 512 and control data and other data from a controller 540.Encoder/modulator 514 processes (e.g., formats, encodes, interleaves,and modulates) the received data and provides a stream of data symbols,{S(k)}. Each data symbol is a modulation symbol for a modulation schemeselected for use. A modulation symbol is a complex value for a specificpoint in a signal constellation corresponding to the selected modulationscheme. A demultiplexer (Demux) 516 receives the stream of data symbols,{S(k)}, and provides these data symbols to the subbands in groups athrough m. Demultiplexer 516 also receives pilot symbols, P(k), whichare modulation symbols known a priori by both the transmitter andreceiver, and provides these pilot symbols to the subbands in group p.Demultiplexer 516 also provides a signal value of zero (a “zero” symbol)for each subband not used for data or pilot transmission.

An OFDM modulator 520 receives the symbols from multiplexer 516 andperforms OFDM modulation on these symbols. OFDM modulator 520 includesan inverse fast Fourier transform (IFFT) unit 522 and a cyclic prefixgenerator 524. For each symbol period, IFFT unit 522 transforms Nsymbols to the time domain using an N-point inverse FFT to obtain atransformed symbol that contains N time-domain chips. Each chip is acomplex value to be transmitted in one chip period. Cyclic prefixgenerator 524 then repeats a portion of each transformed symbol to forman OFDM symbol that contains N+C chips. A symbol period corresponds tothe duration of one OFDM symbol, which is N+C chip periods. OFDMmodulator 520 provides a sequence of N+C time-domain chips for each OFDMsymbol.

A transmitter unit (TMTR) 530 receives and processes (e.g., converts toanalog, filters, amplifies, and frequency upconverts) the stream ofchips to obtain a modulated signal, which is then transmitted from anantenna 532. Controller 540 directs the operation at transmitter 500. Amemory unit 542 provides storage for program codes and data used bycontroller 540.

FIG. 6 shows a block diagram of a receiver 600 capable of performingsubband-based OFDM demodulation to recover data on one or more, groupsof subbands. Again, for clarity, the following description is for datareception on M groups of subbands (i.e., groups a through m) and pilottransmission on one group of subbands (i.e., group p). At receiver 600,the modulated signal transmitted by transmitter 500 is received by anantenna 612. A receiver unit (RCVR) 614 processes (e.g., frequencydownconverts, filters, amplifies, and digitizes) the received signalfrom antenna 612 and provides input samples.

A subband-based OFDM demodulator 620 processes the input samples andprovides recovered data symbols, which are estimates of the data symbolstransmitted by transmitter 500. For the embodiment shown in FIG. 6,subband-based OFDM demodulator 620 includes a cyclic prefix removal unit622, a Fourier transform unit 630, a channel estimator 640, and anequalizer 650. Cyclic prefix removal unit 622 removes the cyclic prefixin each received OFDM symbol and provides a sequence of input samples,{x(n)}, to both Fourier transform unit 630 and channel estimator 640.Fourier transform unit 630 performs partial Fourier transform on theinput sample sequence for each of the M groups of subbands and providesM groups of received symbols, {X_(a)(k)} through {X_(m)(k)}, for the Msubband groups. Channel estimator 640 derives channel gain estimates foreach of the M groups of subbands, based on the sequence of inputsamples, and provides M groups of channel gain estimates, {Ĥ_(a)(k)}through {Ĥ_(m)(k)}, for the M subband groups. Equalizer 650 receives theM groups of received symbols and the M groups of channel gain estimatesfor the M subband groups, performs equalization on the received symbolsas shown in equation (2), and provides M groups of recovered datasymbols, {Ŝ_(a)(k)} through {Ŝ_(m)(k)}, for the M subband groups.

A multiplexer (MUX) 652 receives and multiplexes the recovered datasymbols for the M subband groups and provides one stream of recovereddata symbols, {Ŝ(k))}. A demodulator/decoder 654 processes (e.g.,demodulates, deinterleaves, and decodes) the recovered data symbolstream and provides decoded data to a data sink 656. A controller 660directs the operation at receiver 600. A memory unit 662 providesstorage for program codes and data used by controller 660.

FIG. 7 shows a block diagram of a partial Fourier transform unit 710that may be used to obtain received symbols for one group of subbands.Unit 710 includes a rotator 720, an accumulator 730, a buffer 740, anaddress generator 742, and an N_(c)-point FFT unit 750. Buffer 740stores the N, time-domain values, {g_(m)(n)}, for group m. At the startof each symbol period, buffer 740 is reset (i.e., filled with zeros).

Rotator 720 receives the sequence of input samples. A multiplier 722within rotator 720 multiplies each input sample x(n) with W_(N) ^(mn) toobtain a corresponding rotated input sample {tilde over (x)}_(m)(n), asshown in equation (4). Accumulator 730 performs accumulation for each ofthe N_(c) time-domain values {g_(m)(n)}, as shown in equation (5). Foreach rotated input sample {tilde over (x)}_(m)(n), the current value forthe time-domain value g_(m)(n) for this rotated input sample isretrieved from buffer 740 and provided to an adder 732. Adder 732 addsthe rotated input sample {tilde over (x)}_(m)(n) with the current valueand provides an updated value to a register 734. Register 734 stores theupdated value in the proper location of buffer 740 for the time-domainvalue g_(m)(n).

For each input sample x(n), buffer 740 provides the current value forthe corresponding time-domain sample g_(m)(n) and stores the updatedvalue. An address generator 742 generates the address for buffer 740 andmay be implemented with a modulo counter. At the end of each symbolperiod, after all N input samples for the symbol period have beenreceived and accumulated, FFT unit 750 performs an N_(c)-point FFT onthe N_(c) time-domain values {g_(m)(n)} from buffer 740 to obtain N,received symbols {X_(m)(k)} for the subbands in group m.

FIG. 8 shows a block diagram of an embodiment of channel estimator 640,which can provide channel gain estimates for subband groups a through mbased on pilot symbols received on subband group p. Channel estimator640 includes a partial Fourier transform unit 710 p, a pilotdemodulation unit 860, an N_(c)-point IFFT unit 870, and M partialFourier transform units 880 a through 880 m for the M subband groups.

Partial Fourier transform unit 710 p receives and processes the sequenceof input samples to obtain N_(c) received symbols {X_(p)(k)} for thesubbands in group p. Unit 710 p is implemented in the same manner asunit 710 in FIG. 7, except that a multiplier 722 p within a rotator 720p multiplies the input samples x(n) with W_(N) ^(pn), instead of W_(N)^(mn), and provides rotated input samples {tilde over (x)}_(p)(n). Pilotdemodulation unit 860 multiplies the received symbols {X_(p)(k)} withthe conjugated pilot symbols P*(Lk+p) to obtain the channel gainestimates {Ĥ_(p)(k)} for the subbands in group p. IFFT unit 870 performsan N_(c)-point IFFF on the channel gain estimates {Ĥ_(p)(k)} and providetime-domain channel gain estimates {h_(p)(n)}, and a multiplier 872derotates the time-domain channel estimates {h_(p)(n)} by W_(N) ^(−pn)and provides N_(c) derotated time-domain channel gain values {h(n)}.

Each transform unit 880 receives the N_(c) derotated time-domain channelgain values {h(n)} and derives the channel gain estimates for thesubbands in its assigned group. Each transform unit 880 includes amultiplier 882 and an N_(c)-point FFT unit 884. Within transform unit880 m for group m, a multiplier 882 m multiplies the derotatedtime-domain channel gain values {h(n)} with W_(N) ^(mn). FFT unit 884 mthen performs an N_(c)-point FFT on the rotated channel gain values frommultiplier 882 m and provides N_(c) channel gain estimates {Ĥ_(m)(k)}for group m. M transform units 880 a through 880 m provide M groups ofchannel gain estimates, {Ĥ_(a)(k)} through {Ĥ_(m)(k)}, for subbandgroups a through m, respectively.

Filtering may be performed at various locations along the channelestimation processing path to obtain channel gain estimates withimproved quality. As an example, the rotated input samples {tilde over(x)}_(p)(n) may be averaged over multiple symbol periods prior toperforming the N_(c)-point FFT with unit 750 p. As other examples,filtering may be performed on (1) the received symbols {X_(p)(k)} forthe subbands in group p, (2) the channel gain estimates {Ĥ_(p)(k)} forthe subbands in group p, (3) the time-domain channel gain estimates{h_(p)(n)} for group p, (4) the derotated time-domain channel gainvalues {h(n)}, and so on.

The channel gain estimates for the data subbands may also be obtained inother manners. For example, the channel gain estimates for the subbandsin group m may be obtained by performing (e.g., linear) interpolation onthe channel gain estimates for the subbands in group p.

FIG. 9 shows a block diagram of an embodiment of subband-based OFDMdemodulator 620. Within OFDM demodulator 620, cyclic prefix removal unit622 receives the input samples and removes the cyclic prefix for eachOFDM symbol and provides the sequence of input samples, {x(n)}, toFourier transform unit 630 and channel estimator 640.

Fourier transform unit 630 includes M partial Fourier transform units710 a through 710 m, one transform unit 710 for each of the M subbandgroups. Each partial Fourier transform unit 710 l, where l=a . . . m, isimplemented as shown in FIG. 7. Fourier transform unit 710 l for subbandgroup 1 performs rotation on the sequence of input samples with W_(N)^(ln), accumulates the rotated input samples {{tilde over (x)}l(n)},performs an N_(c)-point FFT on the time-domain values {g_(l)(n)}, andprovides received symbols {X_(l)(k)} for the subbands in group l.Channel estimator 640 is implemented as shown in FIG. 8, processes theinput samples as described above for FIG. 8, and provides channel gainestimates for each of the M subband groups.

Equalizer 650 includes M single-tap equalizers 950 a through 950 m, oneequalizer 950 for each of the M subband groups. Each equalizer 950 l,where l=a . . . m, receives the group of received symbols, {X_(l)(k)},and the group of channel gain estimates, {Ĥ_(l)(k)}, for the associatedsubband group l. Within equalizer 950 m for subband group m, a divider952 m divides the received symbol X_(m)(k) for each subband by thechannel gain estimate Ĥ_(m)(k) for that subband. A slicer 954 m thenslices (i.e., quantizes) the output of divider 952 m and provides therecovered data symbol Ŝ_(m)(k). M equalizers 950 a through 950 m provideM groups of recovered data symbols, {Ŝ_(a)(k)} through {Ŝ_(m)(k)}, forthe M subband groups a through m.

FIG. 9 shows an exemplary receiver architecture in which a single-tapequalizer is used for each data subband. The received symbols andchannel gain estimates may be processed in other manners. For example,matched filtering may be performed on the received symbols with thechannel gain estimates. As another example, log-likelihood ratios (LLRs)may be computed for the received symbols and/or the channel gainestimates, and the LLRs may be processed by a Turbo decoder in aniterative manner.

The techniques described herein for performing partial Fouriertransform, channel estimation, and OFDM demodulation (or simply,“subband-based OFDM demodulation” techniques) can simplify the receiverdesign and provide various benefits. To recover the data symbols for agiven group of subbands, only FFTs of size N_(c) are performed and noFFTs of size N are required. The rotation and accumulation for the Msubband groups can be performed in parallel to avoid extra buffering.Alternatively, one set of hardware may be used to process the M subbandgroups in a time division multiplexed (TDM) manner to reduce hardwarerequirements.

Processing delay is also reduced since the rotation and accumulation foreach subband group may be performed at the sample rate, i.e., on eachinput sample as it arrives at the receiver. The sequence of time-domainvalues, {g_(m)(n)}, for each subband group is available as soon as anentire OFDM symbol is received, without any additional delay. Thechannel gain estimates for each subband group are obtained based on thesame sequence of derotated time-domain channel gain values, {h(n)}.Thus, the channel estimation for the M subband groups may be performedserially without requiring additional buffering.

For simplicity, the subband-based OFDM demodulation techniques have beendescribed for the subband arrangement shown in FIG. 1B. These techniquesmay be used for other subband arrangements. In general, the subbandgroups may include the same number of subbands (as described above) ordifferent number of subbands. Moreover, the subbands in each group maybe selected in some other manners. The only requirement is for thesubbands in each group to be uniformly distributed across the N totalsubbands in order to attain simplification in the FFT by decompositioninto partial FFTs, as described above. For example, if N=4096, group 1may include 32 subbands that are separated by 128 subbands, group 2 mayinclude 1024 subbands that are separated by 4 subbands, and so on. Thepilot subband group may also include the same or different number ofsubbands than the data subband groups. A different subband arrangementmay result in a different phasor W_(N) ^(mn) being used for therotation, a different number of rotated input samples being accumulated,and an FFT of a different size being performed to obtain the receivedsymbols for a given subband group.

The subband-based OFDM demodulation techniques may be used for thedownlink (i.e., forward link) as well as the uplink (i.e., reverselink). For the downlink, transmitter 500 is an access point and receiver600 is a user terminal. For the uplink, transmitter 500 is a userterminal and receiver 600 is an access point. The techniques describedherein may also be used for various OFDM-based systems (e.g., an OFDMAsystem).

For clarity, the subband-based OFDM demodulation techniques have beendescribed for a single-input single-output (SISO) communication system.These techniques may also be used for a multiple-input single-output(MISO) system, a single-input multiple-output (SIMO) system, and amultiple-input multiple-output (MIMO) system. For a MIMO system, oneFourier transform unit 630 is provided for each of multiple (N_(R))receive antennas at the receiver. Each Fourier transform unit 630processes the input samples for an associated antenna and provides Mgroups of received symbols for M groups of subbands for that antenna.Spatial processing is then performed on N_(R) collections of M groups ofreceived symbols for the N_(R) receive antennas to recover the datasymbols. The spatial processing may be performed with a zero-forcingequalizer, a minimum mean square error (MMSE) equalizer, or some othertype of equalizer.

The subband-based OFDM demodulation techniques described herein may beimplemented by various means. For example, these techniques may beimplemented in hardware, software, or a combination thereof. For ahardware implementation, the processing units used to performsubband-based OFDM demodulation may be implemented within one or moreapplication specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, micro-controllers, microprocessors,other electronic units designed to perform the functions describedherein, or a combination thereof.

For a software implementation, the subband-based OFDM demodulationtechniques may be implemented with modules (e.g., procedures, functions,and so on) that perform the functions described herein. The softwarecodes may be stored in a memory unit (e.g., memory unit 662 in FIG. 6)and executed by a processor (e.g., controller 660). The memory unit maybe implemented within the processor or external to the processor, inwhich case it can be communicatively coupled to the processor viavarious means as is known in the art.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method of performing Fourier transform for N_(c) subbands among Ntotal subbands, where N>N_(c)>1, the method comprising: rotating a firstsequence of N input samples to obtain a second sequence of N rotatedinput samples; accumulating the second sequence of N rotated inputsamples to obtain a third sequence of N_(c) time-domain values, whereinthe accumulating is performed for each of N_(c) sets of L rotated inputsamples, where N_(c)·L=N; and performing an N_(c)-point fast Fouriertransform (FFT) on the third sequence of N_(c) time-domain values toobtain N_(c) frequency-domain values for the N_(c) subbands.
 2. Themethod of claim 1, wherein each of the N input samples is rotated bymultiplying the input sample with${W_{N}^{mn} = {\mathbb{e}}^{{- j}\frac{2\pi\quad{mn}}{N}}},$ where n inan index for the input sample in the first sequence and m is an indexfor a subband group that includes the N_(c) subbands.
 3. The method ofclaim 1, wherein each of the N_(c) sets includes every N_(c)-th rotatedinput samples in the second sequence, starting with a different rotatedinput sample.
 4. The method of claim 1, wherein the N input samples arefor one orthogonal frequency division multiplexing (OFDM) symbol, andwherein the N_(c) frequency-domain values are for N, received symbolsfor the N_(c) subbands.
 5. The method of claim 1, wherein the N_(c)subbands include every L-th subband among the N total subbands.
 6. Anapparatus operable to perform Fourier transform for N, subbands among Ntotal subbands, where N>N_(c)>1, the apparatus comprising: a rotatoroperative to rotate a first sequence of N input samples to obtain asecond sequence of N rotated input samples; an accumulator operative toaccumulate the second sequence of N rotated input samples to obtain athird sequence of N_(c) time-domain values, wherein the accumulation isperformed for each of N_(c) sets of L rotated input samples, whereN_(c)·L=N; and a fast Fourier transform (FFT) unit operative to performan N_(c)-point fast Fourier transform on the third sequence of N_(c)time-domain values to obtain N_(c) frequency-domain values for the N_(c)subbands.
 7. An apparatus operable to perform Fourier transform forN_(c) subbands among N total subbands, where N>N_(c)>1, the apparatuscomprising: means for rotating a first sequence of N input samples toobtain a second sequence of N rotated input samples; means foraccumulating the second sequence of N rotated input samples to obtain athird sequence of N_(c) time-domain values, wherein the accumulation isperformed for each of N_(c) sets of L rotated input samples, whereN_(c)·L=N; and means for performing an N_(c)-point fast Fouriertransform (FFT) on the third sequence of N_(c) time-domain values toobtain N_(c) frequency-domain values for the N_(c) subbands
 8. A methodof performing channel estimation in a communication system, comprising:performing a Fourier transform on a sequence of input samples to obtainreceived pilot symbols for a first group of subbands; obtaining a firstgroup of channel gain estimates for the first group of subbands based onthe received pilot symbols; performing an inverse fast Fourier transform(IFFT) on the first group of channel gain estimates to obtain a sequenceof time-domain channel gain values; rotating the sequence of time-domainchannel gain values to obtain a first sequence of rotated channel gainvalues for a second group of subbands; and performing a fast Fouriertransform (FFT) on the first sequence of rotated channel gain values toobtain a second group of channel gain estimates for the second group ofsubbands.
 9. The method of claim 8, wherein the performing the Fouriertransform includes rotating the sequence of input samples to obtain asequence of rotated input samples, accumulating the sequence of rotatedinput samples, in sets of L rotated input samples, to obtain a sequenceof time-domain input values, where L>1, and performing a fast Fouriertransform on the sequence of time-domain input values to obtain thereceived pilot symbols.
 10. The method of claim 8, further comprising:derotating the sequence of time-domain channel gain values to obtain asequence of derotated time-domain channel gain values, and wherein thesequence of derotated time-domain channel gain values is rotated toobtain the first sequence of rotated channel gain values for the secondgroup of subbands.
 11. The method of claim 8, further comprising:rotating the sequence of time-domain channel gain values to obtain asecond sequence of rotated channel gain values for a third group ofsubbands; and performing a fast Fourier transform on the second sequenceof rotated channel gain values to obtain a third group of channel gainestimates for the third group of subbands.
 12. The method of claim 8,wherein the first group of channel gain estimates is obtained bymultiplying each of the received pilot symbols with a conjugated pilotsymbol corresponding to the received pilot symbol.
 13. An apparatusoperable to perform channel estimation in a communication system,comprising: a Fourier transform unit operative to perform a Fouriertransform on a sequence of input samples to obtain received pilotsymbols for a first group of subbands; a pilot demodulator operative toobtain a first group of channel gain estimates for the first group ofsubbands based on the received pilot symbols; an inverse fast Fouriertransform (IFFT) unit operative to perform an inverse fast Fouriertransform on the first group of channel gain estimates to obtain asequence of time-domain channel gain values; a first rotator operativeto rotate the sequence of time-domain channel gain values to obtain afirst sequence of rotated channel gain values for a second group ofsubbands; and a first fast Fourier transform (FFT) unit operative toperform a fast Fourier transform on the first sequence of rotatedchannel gain values to obtain a second group of channel gain estimatesfor the second group of subbands.
 14. The apparatus of claim 13, whereinthe Fourier transform unit includes a second rotator operative to rotatethe sequence of input samples to obtain a sequence of rotated inputsamples, an accumulator operative to accumulate the sequence of rotatedinput samples, in sets of L rotated input samples, to obtain a sequenceof time-domain input values, where L>1, and a second fast Fouriertransform unit operative to perform a fast Fourier transform on thesequence of time-domain input values to obtain the received pilotsymbols.
 15. The apparatus of claim 13, further comprising: a secondrotator operative to rotate the sequence of time-domain channel gainvalues to obtain a second sequence of rotated channel gain values for athird group of subbands; and a second fast Fourier transform unitoperative to perform a fast Fourier transform on the second sequence ofrotated channel gain values to obtain a third group of channel gainestimates for the third group of subbands.
 16. An apparatus operable toperform channel estimation in a communication system, comprising: meansfor performing a Fourier transform on a sequence of input samples toobtain received pilot symbols for a first group of subbands; means forobtaining a first group of channel gain estimates for the first group ofsubbands based on the received pilot symbols; means for performing aninverse fast Fourier transform (IFFT) on the first group of channel gainestimates to obtain a sequence of time-domain channel gain values; meansfor rotating the sequence of time-domain channel gain values to obtain afirst sequence of rotated channel gain values for a second group ofsubbands; and means for performing a fast Fourier transform (FFT) on thefirst sequence of rotated channel gain values to obtain a second groupof channel gain estimates for the second group of subbands.
 17. A methodof performing demodulation in a communication system utilizingorthogonal frequency division multiplexing (OFDM), comprising:performing a partial Fourier transform on a sequence of N input samplesfor an OFDM symbol to obtain a first group of N_(c) received symbols fora first group of N_(c) subbands, where N>N_(c)>1, and wherein thepartial Fourier transform utilizes an N_(c)-point fast Fourier transform(FFT) to obtain the first group of N_(c) received symbols; obtaining afirst group of channel gain estimates for the first group of subbands;and processing the first group of received symbols with the first groupof channel gain estimates to obtain a first group of recovered datasymbols for the first group of subbands.
 18. The method of claim 17,wherein the communication system includes N total subbands, and whereinthe N_(c) subbands in the first group include every L-th subband amongthe N total subbands, where L>1.
 19. The method of claim 17, wherein theobtaining the first group of channel gain estimates includes obtainingtime-domain channel gain values for a group of pilot subbands based onthe sequence of N input samples, rotating the time-domain channel gainvalues to obtain a first sequence of rotated channel gain values for thefirst group of subbands, and performing a fast Fourier transform on thefirst sequence of rotated channel gain values to obtain the first groupof channel gain estimates for the first group of subbands.
 20. Themethod of claim 19, further comprising: performing a partial Fouriertransform on the sequence of N input samples to obtain a second group ofN_(c) received symbols for a second group of N_(c) subbands; rotatingthe time-domain channel gain values to obtain a second sequence ofrotated channel gain values for the second group of subbands; performinga fast Fourier transform on the second sequence of rotated channel gainvalues to obtain a second group of channel gain estimates for the secondgroup of subbands; and processing the second group of received symbolswith the second group of channel gain estimates to obtain a second groupof recovered data symbols for the second group of subbands.
 21. Themethod of claim 17, wherein the first group of recovered data symbols isobtained by dividing the first group of received symbols by the firstgroup of channel gain estimates.
 22. The method of claim 17, wherein thecommunication system is an orthogonal frequency division multiple access(OFDMA) system.
 23. An apparatus in a communication system utilizingorthogonal frequency division multiplexing (OFDM), comprising: a Fouriertransform unit operative to perform a partial Fourier transform on asequence of N input samples for an OFDM symbol to obtain a first groupof N_(c) received symbols for a first group of N_(c) subbands, whereN>N_(c)>1, and wherein the Fourier transform unit utilizes anN_(c)-point fast Fourier transform (FFT) to obtain the first group ofN_(c) received symbols; a channel estimator operative to obtain a firstgroup of channel gain estimates for the first group of subbands; and anequalizer operative to process the first group of received symbols withthe first group of channel gain estimates to obtain a first group ofrecovered data symbols for the first group of subbands.
 24. Theapparatus of claim 23, wherein the Fourier transform unit is operativeto perform a second partial Fourier transform on the sequence of N inputsamples to obtain a second group of N_(c) received symbols for a secondgroup of N_(c) subbands, wherein the channel estimator is operative toobtain a second group of channel gain estimates for the second group ofsubbands, and wherein the equalizer is operative to process the secondgroup of received symbols with the second group of channel gainestimates to obtain a second group of recovered data symbols for thesecond group of subbands.
 25. An apparatus operable to performdemodulation in a communication system utilizing orthogonal frequencydivision multiplexing (OFDM), comprising: means for performing a partialFourier transform on a sequence of N input samples for an OFDM symbol toobtain a first group of N_(c) received symbols for a first group ofN_(c) subbands, where N>N_(c)>1, and wherein the partial Fouriertransform utilizes an N_(c)-point fast Fourier transform (FFT) to obtainthe first group of N_(c) received symbols; means for obtaining a firstgroup of channel gain estimates for the first group of subbands; andmeans for processing the first group of received symbols with the firstgroup of channel gain estimates to obtain a first group of recovereddata symbols for the first group of subbands.